Plant lignocellulosic biomass is used as a renewable feedstock for biofuel production and is a promising alternative to fossil fuel consumption. However, a major bottleneck in biofuel production is the quality of available feedstocks. Available feedstocks have a high resistance (recalcitrance) to being reduced into simple sugars that can in turn be converted into fuel. Therefore, improving the composition and/or digestibility of the raw biomass will have an important beneficial impact on lignocellulosic biofuels production.
Lignocellulosic biomass is mainly composed of secondary cell walls, which comprise polysaccharide polymers embedded in lignin. The embedding of the polysaccharide polymers in lignin reduces their extractability and accessibility to hydrolytic enzymes, resulting in cell wall recalcitrance to enzymatic hydrolysis. Lignin content and saccharification efficiency of plant cell wall usually are highly negatively correlated. See, e.g., Chen and Dixon, Nat. Biotechnol. 25:759-761 (2007); Jorgensen et al., Biofuel Bioprod. Bior. 1:119-134 (2007); and Vinzant et al., Appl. Biochem. Biotechnol. 62:99-104 (1997). However, most attempts at reducing lignin content during plant development have resulted in severe biomass yield reduction (Franke et al., Plant J. 30:33-45 (2002); Shadle et al., Phytochemistry 68:1521-1529 (2007); and Voelker et al., Plant Physiol. 154:874-886 (2010)) and therefore, there are few crops having significant lignin reduction. Although silencing strategies have been used to reduce the amount of lignin in plants, there remains a need for methods of reducing lignin in specific cell and tissue types that reduce cell wall recalcitrance, thus improving the extractability and hydrolysis of fermentable sugars from plant biomass.